8 TABLES AND RESULTS OP THE PRECIPITATION, 



a whalebone scale divided by experiment, so as to indicate tenths and hundredths 

 of an inch of rain. A wooden cylinder is to be inserted permanently in the ground 

 for the protection and ready adjustment of the instrument. 



To ascertain the amount of water produced from snow, a column of the depth 

 of the fall of snow, of the same diameter as the mouth of the funnel, should be 

 melted and measured as so much rain. The simplest method of obtaining a column 

 of snow for this purpose, is to procure a tin tube about two feet long, having one 

 end closed, and precisely of the diameter of the mouth of the gauge. 



From measurements of this kind, repeated in several places when the depth of 

 the snow is unequal, an average quantity may be obtained. 



To facilitate transportation, the larger cylinder is attached to the smaller by a 

 screw-joint. 



A still simpler gauge has lately been devised by the Secretary of the Institution; 

 it consists of a cylindrical tube two and a half or three inches in diameter and 

 nine inches in length, closed with a conical bottom or else provided with a small 

 cylinder projecting from a flat bottom, the object of which would be to give pre- 

 cise indications of the smallest quantities, and, with the same instrument, measure 

 the largest amount which falls in any one shower. But, for ordinary observers, it 

 is thought that nothing is better than a simple cylinder, three or four inches in 

 diameter, and a wooden measuring-rod divided into inches and tenths. Instru- 

 ments of the kind last mentioned are now being distributed among observers. 



To measure thousandths of inches is considered useless labor, since no two 

 gauges can be made to give results agreeing in the hundredths of inches, owing 

 to the irregularity in the rain-fall itself. 



EXPLANATORY REMARKS ON RECORDS. 



The value or precision of the observations collected from such various sources 

 must necessarily have a wide range, and while the greater portion is supposed to 

 come up to a standard precision, a smaller portion partakes more or less of un- 

 certainty, either from want of reliable instruments or from defective observing. 

 In a phenomenon so very irregular as that of the rain-fall, it is difficult to separate 

 defective observations from correct ones; rejections of observations have, therefore, 

 been resorted to very sparingly, and only in such few instances when other records 

 have most undoubtedly proved their unreliability. It may be well briefly to enume- 

 rate the principal sources of error to which observations of the rain-fall were 

 found to be liable : A defective construction of the gauge ; error in the gradu- 

 ation or scale; improper location of gauge in reference to surrounding objects; and 

 allowance, by rule, for water fallen as snow, instead of measuring the melted snow 

 (or hail). Rain-gauges as commonly used, may be supposed liable to no greater 

 error than about two per cent. ; this limit includes error introduced by a defect 

 in the graduation or in the manner of applying a measuring-rod. The most 

 frequent cause of difference in the results by two observers at adjacent stations 

 will generally be found in the manner of exposure of their gauges ; the proper 



